Linear motors are well known in the robotics art and are capable of being moved at relatively high speed across a cooperating surface referred to as a platen. Two-dimensional linear motors typically comprise first and second stepping motors for providing movement of the two-dimensional linear motor in mutually perpendicular directions, each stepping motor comprising a moving member or "forcer" for traversing the surface of a cooperating platen.
The forcer is magnetically propelled along the working surface of a platen. Two-dimensional forcers are described in detail in application Ser. No. 77,112, filed Jul. 23, 1987, now U.S. Pat. No. 4,823,062, issued Apr. 18, 1989 and assigned to the assignee of the present invention. The description of such forcers in the aforementioned patent are incorporated herein by reference thereto.
Magnetic propulsion is accomplished by selectively controlling the power provided to a plurality of electromagnets arranged within the two-dimensional forcer. The electromagnets magnetize themselves to teeth defined by a regular grid array machined into the working surface of the platen and comprised of a plurality of first and second groups of uniformly spaced parallel lines, which first and second groups of lines are mutually orthogonal to define a regular grid matrix. The grid array grooves are preferably filled with a suitable insulating material, the grooves being formed in the surface of a ferromagnetic material or alternatively of a material capable of concentrating magnetic lines of force. The platen with the grooves filled as set forth above thus presents a smooth continuous planar surface.
In order to move two-dimensional linear motors from one position to another with the positioning being precisely at the desired location, it is typical to provide a closed-loop system.
Such a closed-loop control system is described in U.S. Pat. No. 4,823,062 referred to hereinabove. The system described in U.S. Pat. No. 4,823,062 is provided with a platen having grid lines that are separated by distances of the order of 0.04 inches. Although it is possible to obtain finer resolution, it is necessary to accurately form such grid lines in the platen to achieve such resolution, thereby significantly increasing the cost and complexity of the platen. Also, linear motor assemblies do not lend themselves readily to accurate placement through the use of feedback systems.
The two-dimensional linear motor is typically utilized to position a robot arm which moves above a work region. One typical operation is the accurate placement of a microchip upon a printing wiring board with each lead of the chip being precisely placed upon a cooperating mounting solder pad or the like.
The normally encountered design tolerance of the platen coupled with the expansion and contraction of the platen due to changing temperature and climatic conditions, contribute to misalignment of such parts upon a supporting substrate.
The present invention contemplates a method and apparatus for avoiding such costs and complexities.